They were asked late Thursday night to consider additional surveillance and to look out for unattended bags and backpacks, Homeland Security spokesman Brian Roehrkasse said.
Homeland Security officials were keeping close watch on developments related to the terrorist attacks that killed 199 and wounded about 1,200 train riders in Spain.
Based on the current assessment of intelligence "we have no specific indications that terrorists are considering such attacks on the United States in the near term," Roehrkasse said Friday.
The attacks on Thursday have not prompted the United States to raise its terror alert level, which remains at yellow, indicating an "elevated" threat.
A statement claiming responsibility for the attack in Madrid also warned the United States that a major strike is approaching.
An Arabic newspaper said Thursday it had received the claim of responsibility issued in the name of al Qaeda. The claim also said that a major attack against America was "90 percent ready."
The five-page e-mail claim, signed by the shadowy Brigade of Abu Hafs al-Masri, was received at Al-Quds al-Arabi's London offices. It said the brigade's "death squad" had penetrated "one of the pillars of the crusade alliance, Spain," and carried out what it called Operation Death Trains.
"This is part of settling old accounts with Spain, the crusader, and America's ally in its war against Islam," the claim said.
"We announce the good news for the Muslims in the world that the strike of the black wind of death, the expected strike against America, is now at its final stage — 90 percent ready — and it is coming soon, by God's will," the claim said.
Spanish officials thought it more likely that the Basque terrorist group ETA, which has killed over 800 people since 1968 in a campaign for a separate state, was responsible. The explosive used was the kind favored by ETA, and an ETA plot to bomb trains recently was foiled.
But, the attack was far larger than any ever mounted by the ETA before, and unlike most previous ETA operations, it targeted civilians directly and came without warning. ETA has also been weakened by a Spanish offensive against it.
ETA denied responsibility for the bombings.
White House spokesman Scott McClellan said the Homeland Security Department has been working with transit officials on emergency preparedness plans.
"Our efforts have focused on awareness, prevention, response, and recovery to a potential attack," he said.
Amtrak increased patrols of its police force and canine units, spokesman Dan Stessel said. Electronic surveillance of bridges and tunnels was intensified, he said. And the company reinforced its message to Amtrak employees to report suspicious activities to police.
"That gives you another 20,000 sets of eyes," Stessel said. The railroad is continuing to review information received from the FBI and the Transportation Security Administration, he said, but there has been no credible threat against Amtrak or other railroads.
Acting TSA chief David Stone said the agency has been working with public transit systems to close security gaps.
"TSA is very much involved in all risk mitigation plans with trains, metropolitan transit systems and ports," he said.
In New York, Mayor Michael Bloomberg said authorities had moved to increase security in city subways and commuter trains. Uniformed personnel will be present at subway stations and trains, on the Long Island Rail Road, PATH and Metro-North Rail Road trains. Bomb sniffing dogs will also be in use around the region.
"We clearly are focusing, as you would expect us to do, even more resources on the New York City subway system," Bloomberg said at a news conference. "It goes to show we still live in a very dangerous world."
Randy Larsen, a counterterrorism consultant, told the CBS News Early Show that the "term 'winds of death' has people nervous."
"That's why I hope we don't overreact in this country and say we got to put TSA screeners in all the train stations now," he said. "There are 500 Amtrak stations in this country, more than the airports. It's just not possible to protect all of them."
Bruce Hoffman, a terrorism expert at the Rand Corp., said it's much harder to secure transit systems than airports because it isn't possible to closely scrutinize every person in such large crowds.
"If terrorists want to kill a lot of people, public transportation is always the preferred target, because you get a lot of people in the same place at the same time," he said.
Though airplanes continue to be an attractive target for terrorists, he said, the U.S. government's tightening of airport security may have made public transit more vulnerable.
"You harden one target and you shift the threat to another," Hoffman said.
Further, he said, the success of the attacks on trains in Madrid may inspire other terrorists to imitate them.
Trains have proved to be popular targets for terrorists. Last month, a bomb ripped through a Moscow subway car during rush hour, killing 39 people and wounding more than 120. Authorities blamed it on Chechen rebels. In March 2003, an Islamic militant group was blamed for a blast on a train in Bombay killed 11 people and injured 64 more. A doomsday cult conducted the 1995 nerve gas attack on the Tokyo subway, which killed 27 people.
Other recent incidents did not involve terrorists but caused multiple deaths. In February 2003, at least 125 people died when a man who was attempting to commit suicide ignited a fire that engulfed two South Korean subway trains.